Inkjet printers eject or “jet” small bursts of liquid ink from extremely fine nozzles to form an image on an image receiving surface, such as an intermediate transfer surface, or a media substrate, such as paper. The benefits of inkjet printing include low printing noise, low cost per printed page, and the ability to print “full color” images. Full color inkjet printers often utilize a printhead having four ink reservoirs, one for each color required to generate full color images; namely, cyan, magenta, yellow, and black.
Full color and monochromatic inkjet printers can be divided into two categories based on the properties of the ink ejected from the printhead. Aqueous inkjet printers utilize inks formed from a water soluble dye suspended in a polyhydric alcohol. Due to the alcohol base, aqueous inks remain in a liquid phase at room temperature. Therefore, aqueous inks are suitable for printing anytime the user activates the printer. Alternatively, phase change inkjet printers utilize ink that remains in a solid phase at room temperature, often having a waxy consistency. Before the solid ink printhead may eject phase change ink, the printhead must warm the ink to an operating temperature. As the ink reaches operating temperature it melts, or changes phase, allowing the ink to achieve a liquid consistency suitable for ejection by the printhead. The printer maintains at least a portion of the ink at an operating temperature while the printer is activated; however, when the printer is deactivated the ink begins to cool, eventually returning to the solid phase. Before the printer may print an image, the printhead must again melt at least a portion of the ink.
Inkjet printers utilizing either category of ink, often include a printhead having a comparatively small ink reservoir fluidly connected to a larger main ink reservoir. Likewise, full color printers include a printhead having four comparatively small ink reservoirs each fluidly connected to one of four larger main ink reservoirs. Each main ink reservoir receives solid ink, or houses a quantity of aqueous ink supplied by the user. The printer contains circuitry that doses portions of the ink in the main reservoir to the printhead reservoir. A two-stage reservoir system reduces the mass of the printhead, thereby conserving energy and increasing print speed. Additionally, systems utilizing solid ink benefit further because, the small ink reservoir on the printhead avoids the necessity of heating the entire supply of solid ink in the main reservoir for every print job. However, two-stage reservoir systems require the printer to monitor the quantity of ink within the printhead reservoir closely, because if the printhead completely drains the reservoir during the imaging process, the desired image will not be properly formed.
Various methods exist to monitor the supply of ink in the printhead reservoir. One method counts the number of ink drops ejected by the printhead nozzle. Ink drop counting works well and provides the printer's electronic controller with a continuously variable ink level signal ranging from empty to full; however, the system does not provide a real-time monitoring of the actual level of ink in the printhead. Another method utilizes a pair of electrodes immersed within the printhead reservoir. Circuitry monitors the electrical resistance of the ink, and as the ink level changes, so does the resistance measured across the electrodes. Ink resistance monitoring also works well, but only when the printer utilizes an ink with a measurable conductivity. Therefore, further developments in the area of ink level sensing would be desirable.